Biotechnology and genetic engineering have grown in importance in recent years. A basic problem is to detect biological molecules such as DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), proteins, polypeptides, etc. Of great interest for many medical applications are in particular molecules which encode genetic information. Their detection in a sample of a patient enables, for example, pathogens to be detected, thereby facilitating a physician's diagnosis.
Appropriate analytical systems make increasingly use of “biochips” (“lab-on-a-chip systems”) which can be used to detect biological molecules. Different types of biochips are known in which various measurement processes for detecting the molecules are employed. By using biochips, it is possible to construct compact, biological and medical analytical systems.
Embodiments are known in which the biochip is stored in a single-use unit (cartridge), and all steps, from sample processing to detection of the molecules, are carried out within said unit. The operation of such systems is inexpensive, since manual operations are mostly avoided. To carry out the analysis, the cartridge is usually introduced into a read-out and control instrument which controls the analytical process inside the cartridge and reads out measurement data.
For example, it is known that biological molecules can be detected by flushing the biochip with a liquid in which the molecules have been dissolved. This usually requires prior processing for extracting the molecules to be detected from the sample in order to avoid contaminations and facilitate detection. If DNA is to be detected, then it is usually additionally necessary to extract said DNA from cells or viruses present in the sample and amplify said DNA prior to the actual detection. This may be carried out, for example, by way of the “polymerase chain reaction (PCR)”.
In known embodiments, the cartridge harbors a fluidic system which usually includes a plurality of microchannels. These microchannels can transport a liquid sample, for example a blood sample, from an input site to the biochip. In addition, it is possible to work up and process the sample in the channels or in reaction chambers in the cartridge which are connected to the microchannels.
Processing and analyzing the sample usually requires a plurality of different reagents, for example for dissolving the cells (lysis) and for PCR. In the case of compact, cartridge-based systems in particular, the question arises, how such processing reagents can be reacted with the sample. It is known that the required reagents can be transferred from reservoirs arranged in the read-out instrument to the fluidic system of the cartridge. It is moreover known to arrange appropriate reservoir chambers within the cartridge and to store the reagents there. In order to improve durability and storability of the cartridges, biochemical reagents, for example proteins and enzymes, in particular are stored in dry form. This has the additional advantage of it being possible to design a smaller cartridge than when storing liquid reagents.
Usually, reagents stored in dry form are soluble upon contact with water so that they can be reacted with the appropriately delivered sample. However, the fact that the dry reagents dissolve immediately upon contact with the sample or with water, starting the reaction, is not desired in every process step. It is in particular not desired if the sample needs to be washed in a chamber which contains dry reagents prior to the reaction in order to remove contaminations. This is necessary before carrying out a PCR, for example. The washing steps would reduce the concentration of already dissolved dry reagents at the site of the reaction to such an extent that a reaction can no longer take place. It is therefore desirable to dissolve the dry reagents only at a defined point during sample processing or analysis and to start the reaction.
DE-101 11 457 A1 describes a diagnostic device in which required reagents are stored as nonvolatile substances in a microfluidic system. Upon contact with water, the reagents dissolve and are thus available for a reaction or processing. The reagents may be kept ready, for example, in solid form or dissolved in a solid auxiliary substance, for example a water-soluble polymer.
It is known to cover dry reagents with a protective layer of paraffin. This firstly increases the half-life of the reagents and secondly, with the aid of the protective layer, enables the reagents to be released into the microfluidic circulation only at a point in time during the course of the measurement. This may be achieved, for example, by locally increasing the temperature, thereby melting the paraffin. However, the paraffin which now, in the molten state, is a droplet emulsion in the microfluidic circulation of the biochip may impair further processing steps or the analysis itself.